Hybrid electrical-optical cable for overhead installation

ABSTRACT

Hybrid electrical-optical cable for overhead installations for power distribution and for telecommunications, comprising three insulated phase conductors helically wound around a supporting rope. Inside the supporting rope there is at least one optical fibre element fitted in a tubular structure which resists transverse compression, a supporting structure resistant to longitudinal tension being present around the said tubular structure.

The present invention relates to a hybrid electrical-optical cablesuitable for installation along overhead lines for telecommunicationsand for electrical power distribution, particularly at low or mediumvoltage.

There is at present a recognized need to convert the electrical powertransmission and distribution network into a combined network which alsocomprises an optical fibre system for telecommunications.

Various solutions have been proposed in this field for high-voltage(132-400 kV) overhead transmission lines wherein use is made ofnon-insulated conductors suspended between pylons on the top of which aguard wire is installed to protect the line from excess voltages causedby the action of lightning. These solutions require the use, as guardwires, of self-supporting cables which include one or more optical fibreelements for telecommunications. These cables consist of a plurality ofnon-insulated metal conductors, helically wound together in such a wayas to form an inner space which extends longitudinally through the wholecable. Within this space the optical elements are housed, fitted in ametal tubular structure which has the function both of protecting theoptical elements from external mechanical stresses and of draining thecurrents due to atmospheric discharges. Various embodiments of thesecables are described, for example, in patents EP-81,327, U.S Pat. No.4,699,461, U.S Pat. No. 5,123,075 and U.S Pat. No. 5,555,338.

There are also overhead lines for medium or low voltage powerdistribution (generally from 0.4 to 36 kV linked) in which triple-corecables, consisting of three phase conductors, are used, each of theseconductors being insulated with a thermoplastic or cross-linkedpolyolefin layer, which, particularly in medium voltage cables, is inturn surrounded by a metal screen and by a protective sheath made fromthermoplastic material. The three insulated conductors are helicallywound around a metal supporting rope, which provides the mechanicaltensile strength required for the suspended installation of the cable.The supporting rope can be coated by an electrically insulating layerwhen the rope performs the additional function of neutral conductor,particularly when low voltage cables are concerned. Insulated overheadcables of these types are described, for example, in Unified StandardDC4389, 1st ed., February 1994, established by ENEL (Ente NazionaleEnergia Elettrica), Italy.

Overhead lines with insulated cables do not make use of guard wires,since the risk of lightning strike is lower than that encountered withhigh-voltage lines having non-insulated conductors, and is furtherdecreased by the presence of lightning arresters both along the line andat its ends. Moreover, the conductors, being insulated from each otherand from the earth, are not subject to direct-contact voltages.

Consequently, for the conversion of electrical lines for powerdistribution at medium or low voltages with insulated cables into acombined electrical-optical network, it is impossible in practice to useguard wires consisting of self-supporting cables including opticalelements as described above. This would require in fact re-designing andsubstituting the whole line, owing to the addition of an element (theguard wire) which is unnecessary, with consequent unacceptable increasein the costs of installation.

French patent application FR-2,563,042 describes a cable forsimultaneously transmitting medium voltage electric power and highvolume telecommunications wherein three insulated phase conductors arearranged around a central element. The cable would be suitable both forunderground installation and for overhead networks. The central elementcomprises a tube of insulating thermoplastic material containing anoptical transmission module, said tube being surrounded by a layer ofmetal wires which would protect the optical fibers of the module againstmechanical stresses. Externally to the metal wires a sheath is arranged,which is made of lead when the cable is to be installed underground.From page 3, line 27, to page 4, line 1, of that French application itis stated that, when the cable is intended for overhead installations, asheath is generally provided around the thermoplastic tube to protectthe optical fibers against water infiltration, while the metal wires aremade of steel and the sheath surrounding said wires is constituted by arigid plastic material such as polyvinyl chloride or branchedpolyethylene.

According to the Applicant's experience, the cable described inFR-2,563,042 is totally unsuitable for an overhead installation. Infact, in the Applicant's view, in an overhead installation the centralelement containing the optical transmission module must withstand bothlongitudinal stresses and transversal compression forces. While thesteel wires included in the cable disclosed in FR-2,563,042 should beable to resist a longitudinal force, no elements are provided in thatcable to effectively protect the optical element against transversalcompression forces. The only suggestion given in FR-2,563,042 to makethe cable self-sustaining is to replace the external lead sheath placedaround the metal wires with a sheath made of a rigid plastic material.However, the Applicant believes that the external sheath is noteffective at all, since, being placed externally with respect to themetal wires, cannot protect the optical element from the intensecompression forces exerted by the wires when the cable is manufacturedand installed. In fact, the metal wires, as well as the insulatedconductors, when longitudinally stressed, tends to converge towards therope center, thus strongly compressing the optical element placedinside.

Moreover, the cable as described in FR-2,563,042 practically cannot beinstalled on an overhead line, since the central element is placed in aposition which is hardly accessible from the outside. Therefore, thecentral element cannot be used as supporting rope to install the cableoverhead, since it cannot in fact be extracted from the winding of thethree insulated conductors to be suspended to the mooring means of theoverhead line.

The Applicant has now found that it is possible to produce a hybridelectrical-optical cable for overhead installations, comprising athree-phase electrical cable with insulated conductors helically woundaround a supporting rope consisting of an outer structure which resistslongitudinal tension, within which is fitted at least one optical fibreelement enclosed in a tubular structure resistant to transversecompression. In this way it is possible to provide, in a singleself-supporting structure, a combination of insulated conductor elementsand optical fibre elements, ensuring high reliability in operation.

Therefore, in a first aspect the present invention relates to a hybridelectrical-optical cable for overhead installations, comprising threeinsulated phase conductors helically wound around a supporting rope,wherein said supporting rope comprises:

-   -   at least one optical fibre element;    -   a tubular structure containing said at least one optical        element, said tubular structure being resistant to transverse        compression;    -   a supporting structure resistant to longitudinal tension placed        externally to said tubular structure.

According to a preferred aspect, the ratio between the diameter of thesupporting rope and the diameter of each insulated conductor ispredetermined so as to make the rope extractable from the helicallywound insulated conductors. According to a particularly preferredembodiment, said ratio is greater than 0.3, more preferably is from 0.4to 1.5.

According to a preferred embodiment, the insulated conductors are woundaround the supporting rope with a predetermined pitch so as to make thecable self-sustaining. According to a particularly preferred embodiment,said pitch is from 10 to 50 times, more preferably from 20 to 40 times,the diameter of each insulated conductor.

The tubular structure ensures a high degree of protection of the opticalelement, preventing the action of transverse compressive forces on theoptical fibres during the production, installation and operation of thecable, which would cause phenomena of “micro-bending”, with consequentattenuation of the optical signal or even fracture of the fibresthemselves. The transverse compression may be caused both by theinsulated phase conductors, which, under the action of a strong tensileforce, compress the supporting rope and consequently the optical elementcontained in it, and by the supporting structure which, when subjectedto tension, tends to reduce its diameter and consequently the innerspace which houses the optical element. The effects of radialcompression are subsequently amplified by any geometrical irregularitiesof the supporting structure, which may cause localized compressiveforces, which may be very strong, to act on the optical element.

The supporting structure forms the element which enables the cable to besuspended between the sustaining structures (poles, pylons and similar)of the overhead line, since it is capable of withstanding the intensemechanical forces, mainly longitudinal tensile forces, to which thecable is subjected during the installation phase and when it is inoperation. In particular, the supporting structure is capable ofwithstanding the mechanical stresses arising from the weight of thecable itself, from the wind and from the mooring means used to anchorthe cable to the sustaining structures of the overhead line.

According to a second aspect, the present invention relates to anoverhead system for electrical power distribution and fortelecommunications, comprising three insulated phase conductors woundaround a supporting rope, the said cable being fixed between sustainingstructures by mooring means, characterized in that the said supportingrope includes at least one optical fibre element.

According to a further aspect, the present invention relates to a methodfor suspending a hybrid electrical-optical cable to an overhead line,said cable comprising three insulated phase conductors helically woundaround a supporting rope, wherein said method comprises:

-   -   pushing at least one of the three insulated conductors so as to        make the supporting rope accessible from the outside;    -   hooking the supporting rope by a hooking means;    -   extracting the supporting rope by the hooking means from the        wound insulated conductors for a predetermined length;    -   clamping the extracted lenght of the supporting rope by a        mooring means;    -   releasing the supporting rope from the hooking means;    -   suspending the cable to sustaining structures of the overhead        line by the mooring means.

The present invention will now be illustrated more clearly by thefollowing detailed description, provided for further information withoutany limitative purposes for the claim scope, with reference to theattached drawings, wherein:

FIGS. 1 and 2 are schematic representations of transverse sections oftwo possible embodiments of the cable according to the presentinvention;

FIGS. 3-5 show schematically in transverse section three possibleembodiments of the optical fibre element to be used in the cableaccording to the present invention;

FIG. 6 schematically shows a possible way to extract the supporting ropefrom the helically wound insulated conductors to suspend the cable to anoverhead line.

With reference to FIG. 1, the triple-core electrical cable (1),particularly suitable for medium voltage power distribution, comprisesthree single-core elements (10) helically wound around a supporting rope(2) which comprises an optical fibre element (3) (whose specificstructure corresponds to that shown in FIG. 3, described below) fittedin a tubular structure (4) around which a supporting structure (5) ispresent.

Each single-core element (10) comprises, from the inside to the outside,a conductor (11), an inner semiconducting layer (12), an insulatinglayer (13), an outer semiconducting layer (14), a metal screen (15), andan outer sheath (16).

The conductor (11) generally consists of elementary metal wires,preferably made from aluminium or copper, stranded together according toconventional methods, or a single solid aluminium conductor.

The insulating layer (13) is produced by extrusion of a polymercompound, cross-linked or non-cross-linked, having as its base componenta polymer selected, for example, from the following: polyethylene,particularly low-density polyethylene (LDPE), linear low-densitypolyethylene (LLDPE), medium-density polyethylene (MDPE), high-densitypolyethylene (HDPE), cross-linked polyethylene (XLPE); polypropylene(PP); thermoplastic propylene/ethylene copolymers; ethylene-propylenerubbers (EPR) or ethylene-propylene-diene rubbers (EPDM); naturalrubbers; butyl rubbers; ethylene/vinyl acetate copolymers (EVA);ethylene/methyl acrylate copolymers (EMA); ethylene/ethyl acrylatecopolymers (EEA); ethylene/butyl acrylate copolymers (EBA);thermoplastic ethylene/alpha-olefin copolymers; or mixtures of these.Cross-linking, if any, may be carried out by known methods, particularlyby means of peroxide initiators or by means of hydrolysable silanegroups.

The semiconducting layers (12, 14) are made by extrusion of compositionsbased on polymers selected from those indicated above for the insulatinglayer (13), with the addition of carbon black in sufficient quantitiesto impart semiconductive properties.

The metal screen (15) generally consists of metal wires or tapes,longitudinally disposed or helically wound around the core of the cable.

An outer protective sheath (16), consisting of a thermoplastic material,generally polyethylene (PE) or polyvinyl chloride (PVC), is normallyapplied around the screen (15).

With reference to FIG. 2, the triple-core electrical cable (1),particularly suitable for low voltage power distribution, isstructurally similar to that shown in FIG. 1, except that it has neitherthe semiconducting layers (12, 14) nor the metal screen (15).

To impart properties of impact resistance, a layer of expanded polymermaterial (not shown in FIGS. 1 and 2) may be applied around the sheath(16), as described in International Patent Application WO 98/52197. Inparticular, preference is given to polymer materials which have, beforeexpansion, a flexural modulus at room temperature of more than 200 MPaand preferably of at least 400 MPa (measured according to the ASTM D790standard), but in any case not exceeding 2,000 MPa, in order not toincrease the rigidity of the cable excessively. The polymer material maybe selected, in particular, from olefin polymers or copolymers,preferably based on polyethylene (PE) and/or polypropylene (PP) mixedwith ethylene-propylene rubbers. Advantageously, PP modified withethylene-propylene rubbers (EPR) may be used, with a PP/EPR ratio byweight of between 90/10 and 50/50, preferably between 85/15 and 60/40.The degree of expansion of the polymer is highly variable, according tothe specific polymer used and the thickness of the coating which is tobe produced. In general, the degree of expansion may vary from 20% to3,000%, preferably from 30% to 500%. Further details of thecharacteristics of this expanded polymer layer are given in theaforesaid WO 98/52197, the text of which constitutes an integral part ofthe present description.

The tubular structure (4) is generally constituted by a material havinga high mechanical modulus, preferably a metal or a polymeric material.Advantageously, metals or metal alloys having high corrosion resistance,for example aluminium or stainless steel, may be used, or high-moduluspolymers (“technopolymers”) such as polypropylene, modifiedpolypropylene, polybutylene terephthalate (PBT), polyether imides,polyether sulphones, and the like.

The tubular structure (4) may alternatively consist of an expandedpolymer material such as those described in the aforesaid WO 98/52197,in a similar way to that indicated above for the expanded layer whichmay be applied around the sheath (16). The use of an expanded polymermaterial makes it possible to significantly decrease the total weight ofthe cable and to effectively dissipate the energy derived from thetransverse compressive forces such as those described above.

The supporting structure (5) is placed around the tubular structure (4),said supporting structure (5) generally consisting of an armourcomprising one or more layers of metal wires (50), preferably made fromsteel, possibly coated with aluminium or zinc-plated in such a way as toincrease its corrosion resistance, or, alternatively, from an aluminiumalloy. The metal wires are helically stranded around the tubularstructure (4) in such a way as to form a compact structure.

With particular reference to FIG. 2, concerning a low voltage cable, thesupporting structure (5) may be coated by an electrically insulatinglayer (6). In this embodiment the supporting rope (2) performs theadditional function of neutral conductor for the cable. The opticalfibre element (3), whose structure may be selected from those commonlyused for the cores of optical cables, is housed inside the tubularstructure (4). The external diameter of the optical fibre element (3) isslightly smaller than the internal diameter of the tubular structure(4), in such a way as to permit its easy insertion into the tubularstructure (4) while at the same time preventing substantial lateralmovement of the optical element (3) inside the structure (4), whichmight damage the optical fibres.

A first embodiment of the optical fibre element (3) is shown in FIG. 3.This has, in the radially innermost position, a reinforcing element(31), typically made from glass-fibre reinforced plastic. Around thereinforcing element (31) there are disposed one or more tubular elements(32), usually made from PE, PBT or PP, between which are housed theoptical fibres (33), immersed in a buffering filler (34) whose functionis to block any water which may enter the optical element. The tubularelements (32) are also usually embedded in a buffering filler (35). Thebuffering filler which is normally used is a composition based on an oilof the silicone, mineral (naphthenic or paraffinic) or synthetic type,to which is added a viscosity-increasing agent, for example anelastomeric polymer with a low glass transition temperature (for examplepolyisobutene), and, if necessary, a thickening/thixotropic agent (forexample pyrogenic silica), in addition to an antioxidant. The bufferingfiller, if necessary, may also act as a hydrogen absorber; in this case,a hydrogen-absorbing additive, such as carbon palladiate, is dispersedtherein.

Around the tubular elements (33) there is usually present a firstcontaining layer (36) consisting, for example, of a winding of syntheticfibre tapes, for example polyester, having the function of binding theoptical core, and a second containing layer (37), consisting, forexample, of wound tapes of aramid material (for example Kevlar®), havingmechanical and thermal insulation functions.

FIG. 4 shows another example of an optical fibre element (3) which maybe used in the hybrid cable according to the present invention. It has,in the radially innermost position, a reinforcing element (31) on whichis extruded a grooved core (38) wherein are formed external grooves (39)which extend either helically or with an s-z path along the whole outersurface of the said core. The grooves (39) are filled with a bufferingmaterial (34) as described above and house the optical fibres (33). Thegrooved core (38) is then surrounded by a containing layer (36) of thetype described above for FIG. 2.

Finally, FIG. 5 shows a sectional view of another embodiment of theoptical fibre element (3). This element comprises a tubular element (32)containing the optical fibres (33), preferably disposed loosely in thebuffering material (34).

With reference to FIG. 6, the hybrid cable according to the presentinvention may be suspended at predetermined points along the cableextension to an overhead line according to the following method.

Firstly, at least one of the three insulated conductors is pushed so asto partially open the conductor winding and make the supporting ropeeasily accessible from the outside. This operation can be carried out,e.g., by means of one or more wedges inserted between the insulatedconductors.

Then, as shown in FIG. 6, a hook (60) is inserted between the insulatedconductors (10) to clasp the supporting rope (2). To make the extractioneasier, the hook (60) can be mechanically linked to an extremity of acylindrical element (61), longitudinally movable inside a casing (62).The longitudinal movement can be obtained, for instance, by rotating aknob (63) fixed at the other extremity of the cylindrical element (61)opposite to the hook (60). The external surface of the cylindricalelement (61) is therefore provided with a thread (64) to engage it tothe casing (62) whose internal surface is counter-threaded. To link thehook (60) to the cylindical element (61) while leaving themindependently rotatable, a pin (65) is provided in the cylindricalelement (61). The casing (62) may be provided at one extremity withsupporting blocks (66) to be leant against the insulated conductors (10)in order to assist insertion of the hook (60) and extraction of the rope(2).

The hybrid cable according to the present invention can be produced bymeans of a laying machine conventionally used in the cable industry. Toavoid any damage to the optical element, during production it isimportant to apply to the supporting rope a drawing sufficient toconstantly keep the rope in a central position with respect to theinsulated conductors which are wound around it. Consequently, thesupporting rope is prevented from being exceedingly and non-uniformlycompressed by the insulated conductors.

A hybrid cable according to the present invention, whose structure isshown in FIG. 1, was constructed. The electrical cable consisted ofthree single-core elements (nominal diameter: 24 mm), each formed (fromthe inside to the outside) by: an aluminium conductor with across-section of 35 mm²; an inner semiconducting layer (thickness 0.5mm); a cross-linked EPR insulating layer (thickness: 5.5 mm); an outersemiconducting layer (thickness 0.5 mm); an aluminium tape screen,disposed longitudinally (thickness 0.15 mm); a polyethylene sheath(thickness 1.8 mm). The total weight of the electrical cable was ofabout 2.28 kg/m.

The three single-core elements were helically wound (pitch=approximately850 mm) around a supporting rope containing an optical core as thatshown in FIG. 2. The supporting rope, having an overall diameter of12.48 mm, consisted of the optical core (external diameter: 5.5 mm)fitted into an aluminium tube having an external diameter of 8.0 mm anda nominal thickness of 1.25 mm, around which were wound 14aluminium-coated steel wires (Alumoweld), each having a nominal diameterof 2.24 mm. The total weight of the supporting rope was approximately0.48 kg/m.

The supporting rope was subjected to mechanical tests, which revealed avalue of stress at break of 75.4 kN and a value of equivalent elasticitymodulus (for an elongation of 0.3%) of 11000 kN/mm². No attenuation ofthe optical signal (measured by an Optical Time Domain Reflectometer(OTDR)) was observed up to a load of 38 kN. Moreover, compression testswere carried out on the supporting rope, by winding it around a cylinderhaving a diameter of 800 mm with a traction force of about 5,320 kg,thus determining a radial compression of about 13,300 kg/m. After thetest the rope and the aluminium tube did not show any permanentdeformations, and no OTDR attenuation of the optical signal wasobserved.

The measurements so obtained demonstrate that the supporting ropeincluding the optical element is capable of withstanding high tensileforces without causing attenuation phenomena for the optical fibres. Forexample, the hybrid cable described above may be installed in overheadlines with lengths of up to 150 m between pylons, corresponding totensile stresses of not more than 10 kN, with an ample safety margin forthe integrity of the optical element.

1. Hybrid electrical-optical cable for overhead installations,comprising three insulated phase conductors helically wound around asupporting rope, wherein said supporting rope comprises: at least oneoptical fibre element comprising at least one tubular element containingat least one optical fibre; a tubular structure containing said at leastone optical element, said tubular structure being made from a materialhaving a high mechanical modulus to resist transverse compression; asupporting structure resistant to longitudinal tension placed externallyto said tubular structure.
 2. Cable according to claim 1, wherein thematerial of said tubular structure is selected from (a) metals, (b)metal alloys, and (c) high-modulus polymers.
 3. Cable according to claim2, wherein said tubular structure is made from aluminum or stainlesssteel.
 4. Cable according to claim 2, wherein said high-modulus polymerscomprise polypropylene, modified polypropylene, polybutyleneterephthalate, polyether imides or polyether sulphones.
 5. Cableaccording to claim 1, wherein said tubular structure is made from anexpanded polymer.
 6. Cable according to claim 5, wherein said expandedpolymer is selected from (a) olefin polymers and (b) olefin copolymers.7. Cable according to claim 6, wherein said expanded polymer comprisespolypropylene.
 8. Cable according to claim 1, wherein the ratio betweenthe diameter of said supporting rope and the diameter of each insulatedconductor is predetermined so as to make said rope extractable from saidhelically wound insulated conductors.
 9. Cable according to claim 8,wherein said ratio is greater than 0.3.
 10. Cable according to claim 9,wherein said ratio is from 0.4 to 1.5.
 11. Cable according to claim 1,wherein the insulated conductors are wound around said supporting ropewith a predetermined pitch so as to make the cable self-sustaining. 12.Cable according to claim 11, wherein said pitch is from about 10 to 50times the diameter of each insulated conductor.
 13. Cable according toclaim 1, wherein the supporting structure comprises an armour comprisingone or more layers of metal wires helically stranded around said tubularstructure.
 14. Cable according to claim 13, wherein said metal wires aremade from steel.
 15. Cable according to claim 14, wherein said metalwires are made from aluminum-coated or zinc-plated steel.
 16. Cableaccording to claim 13, wherein said metal wires are made from analuminum alloy.
 17. Cable according to claim 1, wherein said supportingstructure is coated by an electrically insulating layer.
 18. Cableaccording to claim 1, wherein said optical fibre element comprises acentral reinforcing element around which one or more tubular elements,containing one or more optical fibres immersed in a buffering filler,are disposed.
 19. Cable according to claim 1, wherein said at least oneoptical fibre element further comprises a central reinforcing elementaround which is disposed a grooved core in which are formed externallyone or more grooves which extend longitudinally along the outer surfaceof said core, said grooves being filled with a buffering filler in whichone or more of said optical fibres are housed.
 20. Cable according toclaim 1, wherein said at least one optical fibre is immersed in abuffering filler.
 21. Optical fibre element comprising at least oneoptical fibre coated by at least a containing layer, said optical fibreelement being fitted in a tubular structure made from an expandedpolymeric material.
 22. Optical fibre element according to claim 21,wherein said polymeric material is selected from (a) olefin polymers and(b) olefin copolymers.
 23. Optical fibre element according to claim 22,wherein said polymeric material comprises polypropylene.
 24. Opticalfibre element according to claim 21, wherein said polymeric material hasa degree of expansion from 20% to 3000%.
 25. Optical fibre elementaccording to claim 24, wherein said polymeric material has a degree ofexpansion from 30% to 500%.
 26. Optical fibre element according to claim21, wherein before expansion said polymeric material has a flexuralmodulus at room temperature between 200 and 2000 MPa.
 27. Optical fibreelement according to claim 26, wherein said flexural modulus is between400 and 2000 MPa.
 28. Overhead system for electrical power distributionand for telecommunications, comprising a cable comprising threeinsulated phase conductors wound around a supporting rope, said cablebeing fixed between sustaining structures by mooring means operating onsaid supporting rope, characterized in that said supporting ropecomprises at least one optical fibre element fitted in a tubularstructure resisting to transverse compression.
 29. Method for suspendinga hybrid electrical-optical cable to an overhead line, said cablecomprising: three insulated phase conductors helically wound around asupporting rope, a tubular structure made of a high mechanical modulusmaterial suitable for containing at least one optical element, said atleast one optical element comprising at least one tubular elementcontaining at least one optical fibre, and a supporting structure placedexternally to said tubular structure, wherein said method comprises:pushing at least one of the three insulated conductors so as to make thesupporting rope accessible from the outside; hooking the supporting ropeby a hooking means; extracting the supporting rope by the hooking meansfrom the wound insulated conductors for a predetermined length; clampingthe extracted length of the supporting rope by a mooring means;releasing the supporting rope from the hooking means; and suspending thecable to sustaining structures of the overhead line by the mooringmeans.